On the ground of recent rapid digitalization of office automation equipments and increasing demand for outputting color images, electrophotographic digital color copying machines and ink-jet/thermal-transfer color printers have been widely diffused as apparatuses for outputting images. For instance, color image data such as texts and landscapes inputted using an inputting apparatus and color image data created by means of a computer, which are based on a visible object taken by a scanner or a digital camera, are printed out using these outputting apparatuses. In these inputting and outputting apparatuses, it is necessary to stably output an image in which colors of inputted color image data are properly reproduced, and a color conversion (color correction) process related to digital image processing techniques plays an important role in realizing this.
Various methods of the color conversion process have conventionally been proposed, including a color coordinate transformation method for converting input image data into uniform color spatial data. As such methods, there are a look up table (hereinafter, will be simply referred to as LUT) method described in publications including the Color Science Association of Japan ed.; “New Color Science Handbook”; University of Tokyo Press; No. 1; pp. 137–149 and “Journal of the Imaging Society of Japan”; Vol. 37; No. 4 (1998); pp. 555–559, and a masking method. In the following descriptions, methods such as correcting a color signal which is inappropriate as an output, converting a color signal in order to change colors themselves, and transforming a color coordinate even if colors themselves are not changed are all termed color conversion.
A direct conversion method which is a sort of the LUT method is arranged in such a manner that after calculating color conversion data with respect to all combinations of input image data, the results thereof are stored in a color conversion table, and then referring to table values in accordance with the input image data, the results are outputted as output image data. This direct conversion method is carried out by accessing the color conversion table so that a required circuit arrangement is simple and the processing is done relatively speedily, and hence this method can be adopted irrespective of nonlinear characteristics.
A three-dimensional correcting method which is also a sort of the LUT method is arranged in such a manner that after calculating table values with respect to the combinations of selected parts of input image data and storing the results in a color conversion table, input image data in proximity to the input image data whose table values have been stored in the color conversion table is worked out by three-dimensional correcting using the table values stored in the color conversion table. With this three-dimensional correcting method, it is possible to work out color conversion values of all combinations of the sets of input image data, even if the number of the sets of input image data whose table values should be calculated in advance is limited. On this account, adopting the three-dimensional correcting method makes it possible to reduce the size of the color conversion table, compared to the direct conversion method.
Moreover, there is a known process termed a memory color conversion process in which an image to which memory colors such as flesh color, sky blue, and vegetable green can be allocated are subjected to a special treatment on the occasion of the color conversion process so that the image is reproduced with preferable colors for humans. When reproducing memory color corresponding data which is inputted as the data of an image to which memory colors are allocated, humans have a preference for color reproduction corresponding to colors similar to those experienced by them at an earlier time, rather than color reproduction precisely corresponding to colors of an image (texts and landscapes) to be reproduced. On this account, carrying out the memory color conversion process enables to output an image with preferable colors. Since a preferable color is different among individuals, an average value of colors preferred by test subjects is usually taken as the most preferable color to be reproduced, and used for apparatuses such as printers.
As an example of the memory color conversion process, there is a Japanese Patent No. 2537997 (registered on Jul. 8, 1996) which discloses color adjustment carried out in such a manner that after adjusting colors only around an area in which memory colors are reproduced and then hard-copying the whole image, the amount of movement in relation to the color adjustment at the barycenter of the area is arranged so as to be large in order to eliminate the discontinuity between colors, and the amount of movement is decreased as distances from the barycenter increase. With this arrangement, natural levels of tone can be kept, only an area having colors similar to memory colors can be freely adjusted by the users, and preferable memory colors can be reproduced.
However, in the conventional art, provided that an object such as a text and a landscape including a part to which memory colors can be allocated is read as an original image and then a color conversion process is carried out so that the original image is reproduced with either precise colors or memory colors and in a visible form such as a printed matter and a displayed image, an image reproduced with precise colors appears unnatural on the occasion of not comparing the image to the original image, while an image reproduced with memory colors appears unnatural on the occasion of comparing the image to the original image. For instance, an image reproduced with precise colors appears natural when compared to the original image, while the reproduced image appears unnatural when not being compared to the original image, because an area which should be reproduced with memory colors is recognized as an area with colors drifted from the memory colors. In contrast, an image reproduced with memory colors appears natural when not being compared to the original image, while this reproduced image appears unnatural as the drift of colors from those of the original image is perceived when compared to the original image.